Membrane contactors have been used for a long time in order to achieve more efficient separation operations, in place of classical processes based on direct contact between two fluid phases, such as gas-liquid absorption, for example, removal of CO2, H2S, or other compounds from gases streams like as natural gas, exhaust gases, etc. A membrane contactor usually consists of a bundle of hollow fibers placed in a membrane module where the fluids flow on each side of the hollow fibers without any direct contact. In addition, membrane contactors provide improved hydrodynamic distribution effects, minimal liquid losses and ease of scale-up.
One problem when using membrane contactors for extraction of a specific gas from a gas stream through absorption by a liquid stream in a hollow fiber membrane contactor is the penetration of the liquid medium into the membrane contactor pores that leads to wetting of the pores and the liquid medium reaching the “medium side”, where the gas stream to be treated is situated and a miscibility of gas and liquid phases occurs. Wetted pores can increase the resistance to mass transfer and severely slow down or even completely inhibit diffusion of the gas to be extracted through the membrane, and as a result decrease the process efficiency.
In order to prevent wetting of the pores, the membrane contactors are made from a hydrophobic material such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), or the absorber side of a respective membrane contactor may be coated with hydrophobic polymers such that the absorption liquid cannot be in direct contact with the absorber side. However, due to the expensive nature of hydrophobic polymers, expensive and long-term non-continuous coating processes, using hydrophobic membrane contactors cause the device to become prohibitively expensive. Recently, methods have been developed for the continuous coating of hollow fibers, but these methods still have various limitations in obtaining a uniform, thin and regular coating in order to achieve high efficiency and high gas permeability without the wetting problem for the hollow fibers.
Hence, there is a need for cost-effective and time-saving methods and systems to achieve a dense, efficient, and uniform coating layer onto the surface of conventional non-expensive hollow fibers that are typically made from hydrophilic materials or materials with lower hydrophobicity. Also, there is a need to coat both sides of a bundle of hollow fiber membranes at the same time continuously or in-situ when they have been placed in a membrane module. Furthermore, there is a need for a method and system to coat both sides of the hollow fiber membranes in a module for two-sided non-wetting purposes, such as liquid-liquid separations or extractions.